Apparatus for film treating

ABSTRACT

Apparatus is disclosed for the surface treatment of a plastic body by exposure to a high intensity voltage accompanied by corona discharge employing a solid state sonic frequency pulse generator.

$18108 atent 1191 Rosenthal et a1. Apr. 24, 1973 1 APPARATUS FOR FILM TREATING 2,969,463 1/1961 McDonald.... ..250 49.5 [75] Inventors: Louis A. Rosenthal, Highland Park; 3 Davis, Somerville, 30th of 31391:; 14 7/1968 Carter ..317/262 A [73] Assignee: Union Carbide Corporation, New OTHER PUBLICATIONS York, N.Y. Principles of Inverter Circuits, Bedford & Hoft, John Wiley & Sons, lnc., New York, London, Sydney, [22] 1971 1964, pp. 165, 166, 184486, 208, 263, 141. ['21] App]. No.: 106,375 Silicon-Controlled Rectifier Inverter With Improved Commutation, W. McMurray & D. P. Shattuck, Related Appllcatmn Data Reprint from Communication & Electronics, Nov. [63] Continuation-impart of Ser, Nos. 862,307, Sept. 30,

[969, abandoned, and Sen Nov 862,412, Sept 3 Prmclples of Inverter C1rcu1ts, Bedf0rd & Hoft, John 19 9,ab d Wiley & Sons, Inc., New York London Sydney 1964 pp. 9092 [52] US. Cl. ...321/45 R, 250/495 GC, 250/495 TC I 51 1m. (:1. ..1-102m 7/48 Primary Examiner-William Show, [58] Field of Search ..250/49.5; 321/45, Alwr'wyPaul Rose, Gsrald oBrien, and Aldo 2 45 .lohn Cozzi [56] References Cited [57] ABSTRACT UNITED STATES PATENTS Apparatus is disclosed for the sur face treatment of a plast1c body by exposure to a h1gh 1ntens1ty voltage ac- 3,514,393 5/1970 Eisby .204/312 companied by corona discharge employing a solid 3,303,406 2/1967 Bedford 321/45 R tate oni frequency pulse generator. 3,294,971 12/1966 Von Der Heide ..250/49.5 3,496,092 2/1970 Fraser ..321/45 X 3 Claims, 5 Drawing Figures Q Patented April 24, 1973 4 Sheets-Sheet 1 INVENTORS g Louis A. Rosemhor Done] A. Davis Patented April 24, 1973 3,729,672

4 Sheets-Sheet 2 INVENTORS Louis A. Rosenfhol y Donald A. Dayis WNEY Patnted A ril 2 1973 4 Sheets-Sheet 5 INV'ENTORS LOUIS A. Rosenthol Donald A. Davis A TTO RNEY Patentd April 24, 1973 4 Sharia-Sheet 4.-

O O O O O O O O O O O O O Q m 8 6 4 2 I000 I200 I400 FREQUENCY HERTZ APPARATUS FOR FILM TREATING This application is a continuation-in-part of our application Ser. No. 862,307, now abandoned, and our application Ser. No. 862,412, now abandoned, both filed Sept. 30, 1969.

BACKGROUND OF THE INVENTION Exposing the surface of a polymer body, such as polyethylene film, to a high voltage gaseous discharge having corona characteristics is known to improve the affinity of the surface for adhesives, inks and other polar substrates. The treatment zone of a typical system comprises a relatively large ground electrode separated from one or more relatively sharp high voltage electrodes by two and preferably three dielectrics. The essential dielectrics are an ionizable gaseous dielectric, normally air, and the polymeric body to be treated. Normally, the ground electrode is covered with a buffer dielectric, such as rubber or a polyester film, which acts to preclude an are from bridging the gap at weak points in the polymer body. The high voltage electrode, which may consist of one or more treater bars in series or in parallel, runs the length of the ground electrode and is in circuit with a high voltage generator.

Most commercial treating systems employ alternating current supplied at frequencies up to 500 kHz or more. Gap voltages'up to kv or more are employed to effectively treat a polymer film which is continuously passed through the gap at speeds up to 500 feet per minute or more. In practice, an energy density-to-film surface of the order of about 1 watt-minute per square foot of film surface or more is sought to achieve good surface adhesion characteristics.

While every component ofa film treating system has come under investigation from time to time, the waveform of the high voltage employed in the treating system has generally been neglected. The spark-gap generators and motor alternators now in use are inefficient and suffer from many inherent deficiencies.

In addition to interfering with radio reception due to the presence of radio frequencies in the spark-gap generator output wave, that generator has a short duty cycle. The range of output power for a given generator is severely limited since the gap breakdown voltage sets the minimum voltage.

The motor alternator, on the other hand, is cumbersome in size and subject to frequent mechanical failure. Further, its output is sinusoidal which is far from the ideal waveform.

In a typical high voltage film treating system, an alternating current line voltage is fed to a high voltage generator and the generator alternating current output is fed through an output transformer to the treating circuit load.

The load should be viewed as a lossy capacitor wherein the electrodes, in their area and spacing, define the capacitance and the dielectric is a composite made up of an air gap, the film and the buffer dielectric all in series. As the corona voltage threshold level is reached, the losses of this system vary in a nonlinear manner. It is the loss component which is effective in treatment and the recognition of the capacitive reactive behavior of the load is important.

The concept of variable frequency has been only recently recognized as the all important parameter for load adjustment and optimization in film treating operations. Looking at the corona treating region as a lossy capacitor system, the power would be proportional to frequencyjust as, for a given input voltage, the current entering a capacitor is linear with frequency. This concept is disclosed and claimed inour copending application, filed of even date herewith, and entitled Film Treating Process.

It has also been recently recognized that the employment of alternating-directional pulse waveform electrical voltage in the sonic frequency range provides the most efficient method of carrying out corona discharge surface treatment with such a capacitive load. Such method is disclosed and claimed in our copending application, entitled Film Treating Method, filed of even date herewith.

SUMMARY OF THE INVENTION The present invention relates to a solid state pulse waveform generator for corona discharge treatment, at a predetermined constant voltage, of plastic bodies comprising: a source of dc power; at least two sequentially-conductive thyristor devices connected in parallel circuit relationship with each other and arranged in series circuit relationship with the load circuit and said source of dc power; reverse-conduction diode means, connected in parallel across each of said thyristor devices, and inductive and capacitive elements arranged in series circuit relationship with each parallel combination of said thyristor devices and diode means and capable of oscillating to effect independent turnoff of said thyristor devices; timing circuit means for sequentially rendering conductive said thyristor devices at a variable frequency; and load circuit means including a step-up, high voltage transformer having inductively coupled primary and secondary windings with a turns ratio capable of developing a voltage in the range of 5,000 to 50,000 volts in said secondary winding; said generator operating in conjunction with corona treating means comprising at least two spaced electrodes, one of which is insulated with a dielectric barrier, connected across said high voltage secondary winding; and wherein the value of said variable frequency of said timing circuit means is adjustable to control the output pulse rate in the sonic-frequency range to optimize the power delivered at said predetermined constant voltage to said treating means.

It has recently been found that a broad range of sonic frequency (20-20,000 Hz) treating voltages may be employed, where frequency is varied to effect surface treatment under optimum load conditions. Accordingly, a treating system providing a broad frequency variation of treating voltage over a range of 20 to 5000 Hz is desired.

In the drawings:

FIG. 1 is a schematic view of apparatus circuitry for the corona discharge treatment of plastic film embodying the invention;

FIG. 2 (a) and (b) are schematic representations of cuit capable of use in the apparatus of the invention; and

FIG. 4 is a graphical representation of the relationship between treating load power and frequency employed for varying electrode lengths in the apparatus of the present invention.

Referring specifically to the system of FIG. 1 of the drawings, a suitable variable direct current source is provided comprising a variable autotransformer having an alternating current supply, the output of which is rectified by a full wave rectifier 12 and filtered by capacitor 14 connected across the output terminals of rectifier 12. The dc voltage output E which is a direct function of the applied autotransformer voltage is fed to the high voltage pulse output circuit 15. Polyphase rectifiers and the like can also be used to provide adjustable dc voltages. It should be noted, however, that the employment of means for varying input voltage and consequent selected output voltage to a desired constant level constitutes merely an apparatus convenience but does not constitute a point of criticality or novelty in the present invention.

The high voltage pulse output circuit 15 comprises a high voltage transformer 16 having a high voltage secondary winding and a low voltage primary center tapped at 18 where voltage E is applied. At least two power thyristors 20 and 22 are coupled at their cathodes and respectively connected at their anodes to the end taps 24 and 26 of the primary of the trans former 16. As described in the article Thyristors: Semiconductors for Power Control by V.W. Wigotsky in Design News, Vol. 22, No. 18, page 26, which is incorporated by reference, thyristors are super switches for electrical power as is their function in the solid state high voltage generator of this invention. The preferred power thyristors are silicon controlled rectifiers but any solid state device or combination of devices which function equivalent to a thyristor or switch can be'used. Ordinarily, a thyristor, particularly a silicon-controlled rectifier in a high conductive state, continues to conduct after the gate signal is removed until the anode current is interrupted or diverted for a time sufficient to permit the rectifier to regain its forward blocking condition.

At least one capacitor 28 is connected across the end taps 24 and 26 of the primary of the transformer and consequently between thyristors 20 and 22.

The high voltage transformer 16 is an important integral part of the high voltage pulse generator circuit. It is center-tapped with end return taps in the primary while the secondary is a high potential winding. Its core must not saturate at operating frequencies and voltages.

At least one pair of diodes 34 and 36 are, respectively, connected at their cathodes to the end taps 24 and 26 of the primary of the transformer 16. The anodes of diodes 34 and 36 are commonly connected to the cathodes of thyristors 20 and 22. Inductor 38 is posi tioned between filtering capacitor 14 and pulse output circuit 15. The diodes 34 and 36 act as anti-parallel or reverse conduction diodes to allow for reversed current flow.

The rate at which power thyristors undergo gating is controlled by a timing circuit 40 which is typically a multivibrator, preferably a free-running, astable, solid state oscillator or a unijunction, astable oscillator which generates trigger pulses of any desired frequency. If coupled with another triggering circuit, monostable and bistable oscillators may also be used. The multivibrator 40 is coupled to the gate of thyristor 20 by a capacitor 42 and resistor 44 and to the gate of thyristor 22 by capacitor 46 and resistor 48 networks, respectively.

Variation of the output frequency of the multivibrator circuit 40 is obtained by the employment of variable resistors 49 and 50 (ganged at 51) which are, respectively positioned in each of the base circuits of the transistors. Such variable control of multivibrator output frequency produces a consequent controllable output from the pulse output circuit 15 which results in output frequency control of power delivered to the treating load circuit 52.

The output of the transformer, thyristor section of the high voltage generator is essentially a pulsed wave of variable frequency. Such output is produced by sequentially gating thyristors 20 and 22 by timing pulses applied to the gates thereof by the timing circuit 40. More particularly, when thyristor 20 is gated or closed, thyristor 22 is maintained in a blocked or open condition and current from the power supply will then flow through the inductor 38 and 1/2 the transformer. The capacitor 28 is across the whole transformer. This series combination of inductor 38 and capacitor 28 oscillate (at a frequency higher than the gating frequency) to provide a single cycle of oscillation. The thyristor 20 is switched off during the time that diode 34 is conducting (i.e., the negative portion of the cycle). Each thyristor is independently turned off by this procedure.

When thyristor 22 is gated, the same sequence occurs using the other half of the transformer in a pushpull manner. By this action, current from the power source alternately flows through the two sides of the transformer primary as the thyristors are sequentially fired.

, Since the direction of current flow through the two halves of the primary is opposed, an alternating, variable frequency, pulse wave output having an amplitude of about [N /N 2 E wherein N is the number of windings on the secondary of the transformer and N is the number of windings on each half of the primary will be created in the secondary having the waveform shown schematically in FIG. 2(a) of the drawings. This voltage is applied to the treater circuit load and produces a treater load current having a pulse waveform as shown schematically in FIG. 2(b) of the drawings.

The waveshape ofthe voltage output from the secondary of the transformer is an alternating pulse superimposed on a residual pedestal. This pedestal is due to the charge remaining on the system capacitance at the end of each pulse. The load current in the treater circuit has the waveform of a series of alternating-directional, sonic frequency single oscillation pulses. There is natural resonant ring due to the transformer following the useful load current burst. This ring does not contribute to corona. Comparing the waveforms of FIGS. 2a and 2b in proper time sequence one can see that the current (2b) is a derivative function of the voltage (2a).

The solid state high voltage generating system disclosed herein is especially suited for use in polymer film treating systems. As shown schematically in FIG. 1, the system as a whole consists of the high voltage generator whose output is connected to the film treating work cell 52 comprising a treater electrode 54 which is normally separated from ground electrode 56 by an air gap 58, the polymeric film 60 and a buffer dielectric 62.

To effectively modify or treat the surface of a polymeric film, the solid state, variable frequency,'high voltage generator must cause a rapid sequence of high voltage gaseous discharges to occur in gap 58 during passage of a polymeric film therethrough.

A modified timing circuit for a treating system generator is shown in FIG. 3 of the drawings. As is there shown, a circuit is disclosed which provides a pulsed output similar to that of the timing circuit embodiment of FIG. 1 but which offers improved operation. The system, as shown, contains a rectifier circuit 63, a timing circuit 64, an isolation stage 65, a multivibrator circuit 67 and an output stage 66.

The frequency control in the embodiment of FIG. 3 is obtained by the variation ofresistive circuit elements 68, 70 and 72. Elements 68 and 70, respectively, control the high and low frequency limits of the timing circuit 64 and, consequently the frequency range limits of the system output. Variation in resistive element 72 offers load (power) control at a constant output voltage by means of frequency control of the output of the system. Circuit element 74 is preferably employed as a relay which provides hard start characteristics in opening, thereby immediately producing drive pulses at the output of the system.

In carrying out treating tests in accordance with the present invention, a high slip polyethylene film 70 inches wide, 1.5 mil thick, traveling at 50 feet per minute, was exposed to the corona discharge provided by the pulse generator of FIG. 1, employing a timing circuit of the type shown in FIG. 3. The voltage fed to the corona generator was maintained at a constant 120 volts dc and the input current varied with variations in I I to the generator is indicative of loading.

In the curves of FIG. 4, the numberassociated with the curve indicates the length (in inches) of the electrode employed for treating.

It is to be noted from the curves that the load is continuously controllable down to essentially zero. No resonances were observed for the frequency range of I575 to 100 Hz. The pulse waveshape has resulted in lower harmonic currents and their associated resonances. It would be expected that in avoiding any resonance absorption, circulating currents and the associated internal heating would be reduced. It was noted that certain components operated cooler with this pulse waveform generator due to the reduction in circulating currents.

Tests were carried out at desired energy densities and the treatment was satisfactory for ink adhesion at commercial levels.

The term high voltage gaseous discharge," as used herein, applies to the discharge phenomenon observed 'during the treatment of polymer films. Although essentially a suppressed are which possesses aspects of corona glow and are discharges, the predominant visual indicia is the corona which has caused the art to term the phenomenon a corona discharge.

To generate the high voltage discharge in the gap 58, the high voltage generator is capable of supplying to a sharp knife-edge electrode at least 2,000 volts ac. Commercial units with larger radius electrodes require from about 5 to 50 KV or more ac which for a dc power supply having an output up to about I20 volts dc will require a transformer having at least 20, preferably or more, windings for each half primary winding. It will be appreciated, however, that the number of secondary windings could vary depending on the magnitude of the selected supply voltage. The solid state high voltage generator should also be capable of providing a power output of from about 5 to about 25 watts per linear inch of electrode 54 to effectively treat the surface of a polymeric film.

Since polymer film treating systems operate at gap film speeds in order of about to 200 feet per minute or more, the astable timing circuit should preferably operate at a frequency of about 20 to 5,000

Hz to closely space the discharges on the film surface. As used herein, the term Hz, or Hertz, is the currently accepted abbreviation for cycles per second.

While not critical to the operation of a polymer film treating system, gap spacings in the order of about onesixteenth to three-sixteenths inch are most commonly employed and contemplated within the ambit of this invention.

In addition to an efficient duty cycle, and the ability to obtain maximum loading conditions through frequency control over an extremely broad range of frequency, the solid state high voltage generator of the invention possesses several characteristics which are deficient in prior generators.

Radio frequency interference is essentially non-existant because the fundamental waveshape is lacking in radio frequency components. This avoids the use of expensive shielding devices and allows its use in areas where regulations have forbidden the use of other generators.

dc Input voltage variation offers a convenience over existing units. Since the timing circuit operates independently of the voltage supply, output voltage is not dependent on the frequency of the timing circuit and any desired output voltage is available at any selected frequency of operation of the timing circuit by variation in dc input voltage. Therefore, for any selected dc input voltage, the output voltage of the generator will be constant and independent of frequency variation.

What is claimed is:

l. A solid state pulse-waveform generator for corona discharge treatment, at a predetermined constant voltage, of plastic bodies comprising: a source of dc power; at least two sequentially-conductive thyristor devices connected in parallel circuit relationship with each other and arranged in seriescircuit relationship with the load circuit and said source of dc power; reverseconduction diode means, connected in parallel across each of said thyristor devices and inductive and capacitive elements arranged in series circuit relationship with each other and in series circuit relationship with each parallel combination of said thyristor devices and diode means and capable of oscillating at a frequency higher than the frequency of timing circuit means to effect independent turnoff of said thyristor devices; timing circuit means for sequentially rendering conductive ble frequency of said timing'circuit means is adjustable to control the output pulse rate in the sonicfrequency range to optimize the power delivered at said predetermined constant voltage to said treating means.

2. A generator in accordance with claim 1, wherein the output pulse rate is variable in the range of 20 to 5000 pulses per second.

3. A generator in accordance with claim 1, wherein said timing circuit means contains resistive frequencyvarying means. 

1. A solid state pulse waveform generator for corona discharge treatment, at a predetermined constant voltage, of plastic bodies comprising: a source of dc power; at least two sequentiallyconductive thyristor devices connected in parallel circuit relationship with each other and arranged in series circuit relationship with the load circuit and said source of dc power; reverse-conduction diode means, connected in parallel across each of said thyristor devices and inductive and capacitive elements arranged in series circuit relationship with each other and in series circuit relationship with each parallel combination of said thyristor devices and diode means and capable of oscillating at a frequency higher than the frequency of timing circuit means to effect independent turnoff of said thyristor devices; timing circuit means for sequentially rendering conductive said thyristor devices at a variable frequency; and load circuit means including a step-up, high voltage transformer having inductively coupled primary and secondary windings with a turns ratio capable of developing a voltage in the range of 5,000 to 50,000 volts in said secondary winding; said generator operating in conjunction with corona treating means comprising at least two spaced electrodes, one of which is insulated with a dielectric barrier, connected across said high voltage secondary winding; and wherein the value of said variable frequency of said timing circuit means is adjustable to control the output pulse rate in the sonic frequency range to optimize the power delivered at said predetermined constant voltage to said treating means.
 2. A generator in accordance with claim 1, wherein the output pulse rate is variable in the range of 20 to 5000 pulses per second.
 3. A generator in accordance with claim 1, wherein said timing circuit means contains resistive frequency-varying means. 